My guess is that the Kelvin timeline was one of those already-existing universes - like with Defiant and the Mirror, they traveled back while shifting over. Alternatively, being within the realm of the event that is enacting the rewrite, there may have been a delay or a protection involved by being within the events gravity/EM field/something sci fi. Until the Kelvin-verse gives me a definitive ending/reset button restoring the Prime, I'm going to go with "always existing alternate." This is all my personal choices and perspectives, of course. To each our own.
That doesn't make it less canon or an option for the writers to explore. The other 20% of time travel stories are just as valid. You also have a variety of different events, like temporal loops (TNG), aliens putting Harry Kim into the "wrong" timeline (but close enough), as well as being able to go back and change the future in various odd ways (VOY was rather notorious for that). Now, that said, we don't see the usual time travel shenanigan effects like we have in time travel rewrites (i.e. missing crewmembers, declarations "this isn't right," efforts to "fix" it). Now, I'm not the smartest guy in the room, but Spock usually is, and is usually the guy to go "We need to do X" in order to fix it. Since he didn't, then it must be one of the random 20% of the times were either a) a new quantum reality was created because of a red matter wormhole or b) Original Spock jumped from one quantum reality to another as well as traveling in time, as demonstrated as a possibility by both the Discovery and the Defiant. Regardless, canon status aside, CBS is treating the Picard show as the 24th century we know from TNG forward, so one must at least look at it from that POV, which means that the 24th century timeline is as destroyed as my TOS DVDs after Abrams' films came out-that is to say, it isn't.
Assignment Earth and Star Trek IV. Kirk's original Enterprise, and Kirk's HMS Bounty in an attempt to go home from the past, are both going to land in the Kelvin Universe, and wonder where they are, and what went wrong.
Chekov doesn’t count: both prime Chekov and nuChekov were born after 2233. So thanks to Nero’s incursion and the timeline changing after that point, nuChekov is probably not even the same person biologically as prime Chekov (which explains why they look so different from each other and why nuChekov is a whiz kid a la Wesley Crusher while prime Chekov isn’t.) Were the Uhuras both born after 2233 as well? If so, the same logic applies, as I don’t think she’s older than Kirk.
The Prime year of Uhura's birth was 2239 and for Chekov 2245, the Kelvin Uhura 2233 and Chekov 2241. They had a 17 year old soldier in combat just to get the character in the film! They should just make them all older and already serving in Starfleet. Nero's incursion gave Kirk and McCoy extra height, changed their eye colour and seemingly changed Sulu's sexuality. What power!
But by then the Enterprise was a cruise ship and as Picard kept insisting they were explorers not soldiers...right?
Though, this is something I found interesting that got pointed out in a commentary I listened to. Chekov is often assigned to the Science station when Spock is not available. So, he may not be "whiz kid" material but it isn't completely unprecedented either. They said the same thing in the Kelvin films too. Pike in 2009 stating "it's a humanitarian and peacekeeping armada" and Scotty arguing with Kirk about the torpedoes in ST ID.
The recruitment officer told a pack of lies..hello Alex Marcus! 'Humanitarian' the very notion is racist!
But I did not say that. What I did say was that works would need to conform to the I.P. owner's requirements. It's not that they'd go and pull existing copies of something from the shelves. (In fact, if a licensed work is submitted to the I.P. owner, passes inspection, goes on sale, and then the I.P. owner decides they don't like something in it, I'd bet good money the contract protects the licensee from having to scrap the issue since they submitted the work to the I.P. owner for inspection in good faith.) It's that future installments would reflect the I.P. owner's requirements. So it's not that DC Comics would have had to pull issues that featured M'Ress from stores; but I am saying that the next issue would have had to get rid of M'Ress. Yep. The lovely thing about time travel is that because it is not real and is not possible, it works however the writers want it to work. I figure Chekov was conceived using in vitro fertilization using preserved ova and sperm cells from his parents. That way, if his parents choose to begin the pregnancy in 2241 in one timeline and 2245 in another, it doesn't matter much and he's biologically the same guy. I see no reason to think Prime!Sulu and Kelvin!Sulu have different sexual orientations.
John Cho says that he played gay in Beyond, but there is no evidence on screen to prove that. George Takei says that he played straight in TOS, but there is no evidence on screen to prove it... Anyone can have a child, shut up.
I don't recall any specific events to indicate Sulu being straight in TOS. To be honest I genuinely tended to see Sulu as gay long before Beyond or even the reboots, but that was just me.
If Takei says he played it with a certain intent, and that Roddenberry had a certain intent, then that is the benchmark for me, personally.